Prior art devices for expelling fluids from flexible bag-like containers, for instance, in zero-gravity environment, as in space travel, have resulted in a problem due to the steady decrease in flow rate of the expelled fluid. An example of such a system is an apparatus for administering intravenous solutions, as may be required in future space travel. In ground-based facilities, reasonably accurate flow may be maintained by means of a gravity induced head pressure. However, where pressure plates or other squeezing means must be utilized to maintain the fluid flow from a container bag, due to the compliant nature of the bag, the surface area against which the external squeezing pressure is applied increases as the bag empties. Thus, a constant squeezing force will result in decreasing internal pressure in the container bag and resultant decreasing flow rate.
Accordingly, the main object of the present invention is to provide a novel pressure modulating valve which maintains a constant internal pressure in the container with a resultant constant flow rate.